1. Field of the Invention
The present invention relates to an antenna apparatus which reduces the side lobes without increasing the beam width of the antenna pattern.
2. Description of the Related Art
The antenna pattern of general type of antenna including receiving antennas is improved as the beam width and the side lobes thereof, indexes of a good antenna pattern, are reduced.
A known antenna device comprising two antennas arranged apart from each other utilizes the multiplication principle of the directional characteristics of antennas in order to reduce the beam width of the antenna device. According to this principle, the combined pattern of the antenna device is obtained by multiplying the pattern of the individual antennas by the array factor of the antenna device. FIG. 1 schematically illustrates such an antenna device. The antenna device comprises first and second antennas 101, 102 which are arranged so that the distance a between the centers of the first and second antennas 101, 102 is equal to or greater than the aperture length b of each of the antennas 101, 102. By this arrangement, the angle of the first zero point of the array factor of the antenna device becomes smaller than the angle of the zero point of the pattern of the individual antennas 101, 102, thereby reducing the beam width of the antenna device.
However, the conventional art, including the above-described method for reducing the beam width, fails to reduce either one of the beam width and the level of side lobes, that is, indexes of a good antenna pattern, without increasing the other. According to the conventional art, a reduction of the beam width results in an increase of the level of side lobes, and a reduction of the level of side lobes results in an increase of the beam width.
This drawback of the conventional art may cause problems. For example, if the side lobe level of a radar antenna is reduced and, therefore, the beam width thereof is inevitably increased, the resolution of the radar deteriorates, thus reducing the object distinguishing power of the radar. In such a case, the radar may fail to distinguish a plurality of objects and, instead, recognize them as a single object. If the beam width of a radar is reduced and, therefore, the side lobe level is inevitably increased, the radar may make an error in determining whether there are any objects in the direction of the beam (the observation direction). More specifically, if no object exists in the observation direction but an object exists in the direction of the thus-enhanced side lobe, the radar may determine that there is an object in the observation direction.
Because neither one of the beam width and the side lobe level can be reduced without increasing the other, the conventional art merely provides a compromise solution based on distributions, for example, Chebyshev distribution, in which the minimum beam width is obtained with respect to a certain side lobe level, or in which the minimum side lobe level is obtained with respect to a certain beam width.